Subject: Re: Interaction between DHCP and DNS

From: Mark Stapp <[email-masked]>
Date: Wed, 29 Mar 2000 12:26:24 -0500

Jin,

Thanks for your questions; I've replied inline.

At 02:49 PM 3/28/00 +0100, Yang Jin-JINYANG1 wrote:

[...]

>Assuming Host-A's IP address is assigned dynamically by DHCP and DNS is 
>updated to reflect this new mapping, how would the following scenarios work:
>
>(1) After Host-A log-off, will DNS be informed?

That depends. If the client at A performed its own DNS update, it may try 
to perform another DNS update as it goes down in order to remove DNS 
information. But even if it tries, it may not succeed: it may not be able 
to reach the DNS server at that moment, or it may be shut down so rapidly 
that it doesn't have time to attempt an update.

If the DHCP server performed DNS updates for the client, the server will 
not even know that the client has logged off unless the client sends a 
DHCPRELEASE message as it shuts down. That may not be desirable in all 
installations: it may be costly to the DHCP and DNS servers to remove lease 
bindings and DNS records each time a client reboots, or is shut down 
overnight. Since the DHCP server is likely to be available most of the 
time, it's probably easier for it to do the house-cleaning as leases expire 
or are released, but that's not a requirement, and different sites may 
develop different policies.

There is an administrative issue, however. If the client is *permanently* 
shutting down, and not just being put to sleep overnight, many 
administrators will want to remove its DNS name so that "stale" DNS data 
doesn't accumulate indefinitely. The draft tries to describe some of the 
administrative policies that I could imagine, or that were offered by others.

The dhcp-dns draft doesn't attempt to tell administrators what policy to 
apply in their nets, it just tries to establish a consistent way of adding 
information that they and their dhcp clients and servers can use to apply 
_some_ policy.

>(2) Host-B gets A's IP address via DNS and cached it locally. Then Host-A 
>log-off but Host-B does not know and continually sends packets to A's IP 
>address. Is this a concern?

DNS isn't really a database of currently connected hosts, of course, so the 
presence of a name in the DNS isn't a guarantee that a host is present and 
reachable on the net. My statically-configured workstations may not always 
be connected, and they may not always be reachable from every other host. 
So no, in general the concerns aren't much different for dhcp and non-dhcp 
clients.

The issue of dns ttls has been raised in the course of dhcp and dns 
discussions about the dhcp-dns draft. The draft doesn't contain strict 
rules that must be applied, but it does discuss dns ttls, and suggest a 
couple of possible ways to establish reasonable ttls.

Regards,
Mark